The invention is in the field of bus systems with a bus interconnected to a coupling module to form a ring structure, whereby a linear structure of the bus is achieved through a partition in the coupling module. The bus system is also equipped with a monitoring device contained in the coupling module, which monitors the bus for identical conditions on both sides of the partition and closes the partition via a controllable switch device in the case of a discrepancy.
Such a bus system is known from Siemens Energy Production KWU: Instrumentation Technology Energy Production: Products and Services: Bus System SIMATIC NET, online on the Internet: URL: http://www.siemens.de/kwu/d/foa/1/products/prod160.htm (as of Jan. 29, 1999). In this known bus system, the bus stations are connected via star connections to a bus. In order to increase the bus system""s availability and error protection, the bus is interconnected with the star connections to form a ring structure, whereby a linear structure is achieved by a partition in one of the star connections. Signals (telegrams) coming from both directions are monitored at the partition. If the signals come only from one direction, it must be assumed that there has been an interruption of the bus. In this case, the partition is closed by a controllable switch, thus reforming a closed linear structure of the bus, and all bus stations can communicate with each other again via the bus.
Monitoring the bus for identical telegrams on both sides of the partition is relatively difficult and can be associated with delays due to the necessity of evaluating the telegrams. In addition, the monitoring function is only effective while telegrams are being transmitted via the bus.
One object of the present invention is to guarantee an especially simple and stable bus redundancy.
In accordance with the present invention, these and other objects are solved by providing a bus system and method wherein power is supplied to the bus stations and a monitoring device records and compares the supply voltage on both sides of the partition.
According to the system and method of the invention, in bus systems that transmit both telegrams and supply voltage to the bus stations on the same bus, it is the supply voltage, and not necessarily the telegrams, that is monitored and compared on both sides of the partition in order to detect a bus interruption. The monitoring of the supply voltage in accordance with the invention is simple, malfunction-proof and even possible at times when no telegrams are being transmitted. In principle, the invention is appropriate to all buses in which the power supply and data transfer takes place via the same cable, regardless of whether the telegrams are modulated to the supply voltage for the bus stations, or if the telegrams and the supply voltage are transmitted via separate leads, as for example in the optical transfer of data.
In order to notify the bus stations, especially the bus master (active bus station), if there is a bus interruption, the coupling module preferably contains a bus station device that is linked to the bus The monitoring device directs the bus station device to send an error message to the bus in the event that a discrepancy between the supply voltage on both sides of the partition is detected. It is advantageous for the error message to contain information about the side of the partition on which the supply voltage was found to fall below the minimal value, so that it is easier to ascertain the bus interruption location in relation to the position at which the supply voltage is fed into the bus.
The controllable switching device that serves to close the partition is preferably also controllable by a command that is transferred to the bus station device via the bus. The bus master can then specifically open and close the partition to test the coupling module and to put the repaired bus back into operation following an interruption. Alternatively or preferably, the coupling module can also be equipped with a manually operable control element and/or a separate control signal input for operating the controllable switching device.